West Nile Virus

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West Nile virus is a mosquito-borne virus that can infect humans, birds, horses, and other mammals. In most humans, West Nile virus infection causes a mild or moderate, short-lived flu-like illness, or causes no symptoms at all. However, in some cases, particularly among persons 50 years of age and older, it can cause serious neurological diseases such as encephalitis, meningitis, or paralysis. West Nile virus first appeared in North America in New York City in 1999. Since then, the virus has spread across the continental United States.